Epitaxial Growth and Optical Properties of Laser Deposited CdS Thin Films

Cadmium Sulde (Cds) thin lms were synthesized on quartz substrates using infrared pulsed laser deposition (IR-PLD) technique under high vacuum (~ 10 − 6 Torr). X-ray diffraction was used to evaluate the structural features. According to X-ray analysis the deposited CdS lms are crystalline and have a favored orientation on a plane (110) of an orthorhombic system and the peak intensity and the average crystallite size increases with increasing the lm thickness. After annealing at 300 o C the orthorhombic phase transformed into predominant hexagonal phase and the same result was obtained by SEM photographs. Spectrophotometric measurements of transmittance and reectance of the Cds lms were used to derive optical constants (n, k and absorption coecient α). The optical band gap energy was found to be 2.44 eV. The plasma plume formation and expansion during the lm deposition have been discussed. The photocurrent response as a function of the incident photon energy E (eV) at different bias voltages for different samples of thicknesses (85, 180, 220 and 340 nm) have been studied, indicating that the photocurrent increases by increasing both the lm thickness and photon energy with a peak in the vicinity of the band edge. Thus, the prepared Cds lms are promising for application in optoelectronic eld.


Introduction
Cadmium Sul de CdS, is one of the composite semiconductors (II-VI). Which have a high direct energy gap (2.42 eV) at temperature (300 kelvin), where could be used in solar cells, light-emitting diodes, thin-lm transistors, and optoelectronics devices. In light of this, Cadmium Sul de (CdS) have attracted and attention of many researchers in the scienti c and industrial led. To produce a thin layer of Cadmium Sul de (CdS), There are a variety of fabrication techniques used such as chemical bath deposition (CBD) [1,2], electrodeposition [3], chemical spray pyrolysis (CSP) [4], successive ionic layer adsorption (SILA) [5], Spin-Coated [6 ],vacuum evaporation [7], screen printing [8], ash evaporation [9], sputtering [10], molecular beam epitaxy (MBE) [11], and pulsed laser deposition (PLD) [12,13]. Among these techniques, pulsed laser deposition (PLD), which have the following advantages (I) simple and easy: where a laser beam vaporizes a target surface, resulting in a lm with the same composition as the target. (II) Many materials can be deposited in a variety of gases at a variety of pressures. (III) cost-effective: a single laser can power many vacuum systems. (VI) In 10 or 15 minutes, highquality samples may be reliably prepared. (IV) scalable. In general, easier to obtain the desired lm stoichiometry for multielement materials using PLD than with other deposition technologies. In recent years, pulsed laser deposition (PLD) technique has been widely developed and become a suitable technique for the deposition of high-quality compound semiconductor thin lms [14].
In this work, we will prepared thin lms of Cadmium Sul de CdS with different thickness by using pulsed laser deposition technique. where the laser ablation deposition system used for CdS thin lms preparation consists of the evaporation source, which is the fundamental wavelength for Nd: YAG Laser (1064 nm), the evaporation process depends on laser parameters such as laser energies and pulses duration [15]. The deposition chamber: is made of stainless steel, nally pumping system: which is composed of a rotary mechanical pump connected with in series with a turbomolecular pump, This combination of the two pumps can reach a high vacuum pressure [16]. The lm thickness can be controlled to a single atomic layer by ne adjusting the pulse number. Those unique merits make PLD suitable for the deposition of various highquality thin lms [17] The objective of the present work is synthesis of transparent and conductive CdS thin lms with high quality, for optoelectronic devices applications.

Preparation CdS thin lms
Cadmium Sul de (CdS) thin lms were deposited in a vacuum chamber using 1064 nm Nd-YAG laser (Model PL-7010, CA 95051) produced by Continuum. Cadmium Sul de (CdS) is a semiconducting compound (II-VI) in the powder form with purity of 99 % (BDH). The CdS targets were made in the form of pellets with dimensions 10 mm in diameter and 3 mm in thickness by grinding the material to be homogeneous ne powder and then compressed under high pressure of 8 x 10 5 N/m 2 . A CdS targets was mounted on a rotating target stage. The substrates were quartz glass placed 60 mm apart from the target surface. A schematic view of the lm deposition setup is shown in Fig. (1).
Six samples of CdS thin lm with different thickness were fabricated at room temperature. The vacuum chamber pressure was maintained ~ 10 − 6 Torr during the entire deposition process. The laser Fluence was 4J/cm 2 and repetition rate was 10 Hz. The thickness of these six samples ranging from 60 nm to 340 nm.

Characterization techniques
The lm thickness was measured by the multiple beam Fizeau fringes method [18,19]. The X-ray diffraction patterns for CdS in the powdered and in thin lm form for sample thickness 340 nm for as-deposited and after annealing at 300 o C were recorded automatically using Philips, Cu K α radiation with λ = 1.54 Å. while the software Diano Corporation XSPEX (Woburn, Massachusetts 01801 U.S.A) and Fe ltered Co radiation with λ = 1.79 Å were used for X-ray diffraction patterns and line broadening of (110) plane for different thicknesses of CdS thin lms. The detector is scintillate counter with a dead time of less than 10 − 6 sec. The X-ray tube was energized at 45 kV and 10 mA.
Scanning electron microscope type Jeol 100S with 60 kV operating voltage and a resolution of 50Å with zero angle of sample inclination, was used for study morphological surface of CdS lms Spectrophotometric method was used for measuring the optical transmittance T, and re ectance R of CdS thin lms in the spectral range of 200-2500 nm. UV/VIS/NIR double beam spectrophotometer (JASCO model V-570) was used.

Structural analysis
The XRD patterns of the CdS powder and thin lms deposited under vacuum (10 − 6 Tor) are shown in Fig. 2. for CdS in a powder form as represented in Fig. (1a) shows that there are two different phases (hexagonal and orthorhombic). The predominant phase is the hexagonal phase, as indicated from the peak intensities of the hexagonal one. The interplanar spacing (d hkl ) were calculated using Diano Corporation XSPEX software and compared with those given in the (JCPDS) card le. Data representing the interplanar spacing (d hkl ) and relative intensities I/Io for different (hkl) planes are given in Table (1). This indicates a polycrystalline nature of the powdered sample with lattice constants a = 4.136 Å and c = 6.713 Å for hexagonal form (ICDD 77-2306).
The X-ray pattern of the thin lm shows a well-developed crystallographic texture (preferred orientation), where only two peaks namely (110) and (220) are clearly identi ed and their d values are related to the orthorhombic phase (ICDD 43-0985) which agrees with that reported by V.V.Yakovlev et al. [20]. This means that the crystallites of the lm are oriented in such a way that the (110) planes are parallel to the substrate surface. The data representing the interplanar spacing (d hkl ) are given in Table (1). Figure (2C) shows the X-ray diffractograms for patterns of CdS thin lm of thickness (340 nm) before and after annealing.
When the sample was annealed at 300 o C for two hours, X-ray pattern indicates that the intensity of the (110) plane was decreased and the (220) plane disappeared, and other peaks of the hexagonal form appear with considerable intensity as in Fig. (2b). This can be explained as that a part of the orthorhombic form is transformed into hexagonal one. The data in Table (1) show that the majority of the structure is hexagonal phase (ICDD 77-2306) with its characteristic peaks, and this is consistent with the SEM photographs of CdS as shown in Fig. (5a,b).
The resulting X-ray diffractograms for as-deposited CdS thin lms with different thicknesses ranged from (60-340 nm) are represented in Fig.(3). All lms showed preferred orientation parallel to [110] direction and the peak intensity increases with increasing lm thickness where no shift in the peak position (2θ = 37 o ) is observable X-ray line broadening of the (110) re ection of the as-deposited CdS thin lms with different thicknesses is represented in Fig. (3). The crystallite size and microstrain as a function of lm thickness are determined and represented in Fig. (4: a, b) respectively. From Fig. (4a), it is observed that the average crystallite size increases with increasing the lm thickness. The observation of A.Amith [21] and J.B.Wilson [22] regarding the increase of crystallite size with increasing of the lm thickness agrees well with our results. Large number of Cd 2+ and S 2+ ions increase with increasing the lm thickness and get adsorbed on the substrate and lling the voids leads to an increase in the value of crystallite size and the defects in the lattice are reduced which in turn reduces the microstrain [23] as shown in Fig. (4b).
Scanning electron microscope (SEM) micrographs represented in Fig. (5a, b) shows the orthorhombic form for the asdeposited CdS thin lms.

Optical Properties.
The transmittance (T) and the re ectance (R) for each lm were measured by spectrophotometric method at normal incidence of light in the spectral range 200-2500 nm.
The transmittance for all CdS lms decreases with increasing the thickness while increases with increasing wavelength.
The spectral behavior of refractive index n(λ) and absorption index k(λ) for the homogeneous CdS lms represents Figure (7). The obtained results of both n(λ) and k(λ) show that these parameters are independent of the lm thickness in the thickness range (60-340 nm). The uctuation in both n(λ) and k(λ) can be attributed to the experimental errors in T, R and the lm thickness (d). The curve behaves like a normal dispersion above λ = 1500 nm and shows anomalous dispersion towards a shorter wavelength. The peak that appears in the spectral distribution of the refractive index is due to a rapid increase in the absorption mechanism in the fundamental absorption edge [24]. No variations in (n) with the lm thickness could be observed in the utilized lm thickness ranges. The error in calculated values of (n) was estimated to be 3% due to experimental error for measuring T, R = ± 0.01%, and lm thickness = 2%.
The absorption coe cient, (α) of as-deposited CdS thin lms was calculated using the equation: Where k; the mean value of the absorption index at a particular wavelength. The spectral behavior of the absorption coe cient (α) versus photon energy (hν) and the (αhυ) 2 as a function of hυ for as-deposited CdS lms are shown in  Fig. (9). One linear relation is obtained having a slope = 0.5, indicating and con rming the allowed direct optical transitions.

Plasma Plume and Spectral Pro les
Plasma produced by the focusing of laser beam on solid targets became the object of intensive theoretical and experimental studies immediately after high power lasers became available [26].
Different diagnostic techniques, such as optical spectroscopy [27,28], Laser-induced uorescence [29], have been used in attempts to characterize the plasma as it expands either in a vacuum or in an ambient atmosphere. The laser beam is concentrated to a spot of 0.5 mm in diameter on the CdS target surface with an angle of incidence 0°. Expansions of the plasma plumes were observed and photographed by using a pinhole digital camera, when the Nd: YAG laser beam of wavelength 1064 nm impinges onto a CdS target surface, the plasma expands perpendicularly to the target surface independently of the angle of incidence.
After a certain number of laser shots, the depth of the hole increases with the number of laser pulses. By rotating the target to get a new target surface, the direction of the plasma plume turns increasingly towards the incident laser beam, as shown in Fig. (10) a. By increasing the hole depth, the observed plasma plume outside the hole becomes shorter, and the intensity of the emission decreases, as indicated in Fig. (10) b, c. When the laser beam interacts with the plasma plume, some of its energy is absorbed by the atoms, exiting them. Electrons are pushed into high energy states, only to later fall back down to lower states. When this transition occurs, radiation is emitted at a speci c frequency. The emitted spectra were recorded from a side window using a monochromator attached with a charge-coupled device (CCD) camera controlled by a computer program through an optical ber. Spectra were taken at different distances from the target surface. Figure (11)a-e shows the emission spectra from a CdS target. There is a broadening in the emission lines due to both frequent causes, namely the spectrometer having 0.5 nm resolution, and physical causes such as a "Doppler shift" due to atomic movement and the effect of atomic collisions in the target plume.
The image pro le and three-dimensional pro le of the spectral line 477.16 nm are shown in Fig. (11) a, b.
Unfortunately, most of the emission lines of the Cd 2+ made up the plasmas lies in the ultraviolet region of the electromagnetic spectrum outside the range of our spectrometer but most of the S 2+ lines are detected and were compared with the known emission lines [30] as represented in Table (2).

Photoconductivity Response
The spectrum of the CdS thin lms by a different bias voltage (1-6 V) was characterized with the photon energy, E (eV), as shown in Fig. (12)a-d. The samples for each thickness 85, 180, 220 and 340 nm, respectively, display increasing of the photocurrent response by raising the incident photon energy E (eV), with a peak in the vicinity of the band edge. This can be attributed to the generation of a greater number of free charge carriers in the bandgap region, and this is probably connected with the grain boundary, which induces an electric eld due to the spatial distribution of electrons in the grain [31]. The optimized as-deposited CdS thin lm at thickness 340 nm exhibits the high energy at voltage 6V and photocurrent approximately 80. In darkness, the photoconductor resistance is very high, and when a voltage is applied, only a small dark current was observed. When light is incident on this photoconductor, a current (I) ows due to the generation of more number of charge carriers as illustrated in the gure below.
Here are the basic principles of the photoconductive effect [32]: (Ι) Directly beneath the conduction band of the CdS crystal is a donor level and there is an acceptor level above the valence band. In darkness, the electrons and holes in each level are almost crammed in place in the crystal and the photoconductor is at high resistance. (ΙΙ) When light illuminates the CdS lm surface and is absorbed, the electrons in the valence band are excited into the conduction band. This creates pairs of free holes in the valence band and free electrons in the conduction band, increasing the conductance. (ΙΙΙ) Furthermore, near the valence band is a separate acceptor level that can capture free electrons only with di culty but captures free holes easily. This lowers the recombination probability of the electrons and holes and increases the number of electrons in the conduction band for n-type conductance. Until the carriers generated in (ΙΙ) and (ΙΙΙ) recombine, electrons are injected from one electrode and pulled out by the other. When these carriers last longer and they move more, the conductance increases greatly.

Conclusion
The X-ray analysis of the prepared CdS thin lms shows that all lms crystalline with an orthorhombic system and have preferred orientation in (110) direction. The peak intensity of the plane (110) increases with increasing the lm thickness.
After annealing at 300 o C for the sample of thickness 340 nm, the X-ray diffraction pattern indicates that the orthorhombic phase is transformed into predominant hexagonal phase, and this was con rmed by SEM photographs. The average crystallite size of the lms was growth with increasing lm thickness due to the increase in the number of Cd 2+ and S 2+ ions with increasing the thickness, get adsorbed on the substrate and lling the voids. It increases crystallite size value, and the defects in the lattice are reduced, which reduces the microstrain in the samples. The cadmium Sul de CdS follow the direct allowed transition with E g = 2.44 eV. The direction of the plasma plume expansion is perpendicular to the target surface, and the plume length decreases with increasing the number of incident laser pulses. The photocurrent is increasing with expanding the lm thickness, and this is often due to the growth of crystallinity of thin lms with expanding the lm thickness.     Transmittance and re ectance curves of (a) homogeneous CdS thin lms (b) inhomogeneous CdS thin lms.   The relation between (αhν)2 and photon energy (hν) for CdS lms.

Figure 9
Relation between ln (α) and ln(hν-Eg) for CdS lms.  Plasma with different spectral line during the deposition.  Photoconductivity response spectra at different bias voltage for CdS thin lm with different thickness(a)85nm, (b)180nm, (c)220nm, (d)340nm.

Figure 14
Dark and illuminated I-V characteristics at different wavelengths for CdS lm with thickness 340 nm. Carrier generation by light excitation